The present invention relates to a cooling apparatus capable of conducting cooling so as to obtain the target temperature of a semiconductor device that comprises a plane-like surface, generates heat when electric current is passed therethrough, has the temperature thereof rising to above the target temperature, and has the temperature of the surface rising.
A burn-in apparatus for semiconductor devices (referred to hereinbelow as “devices”) is generally known in which a multiplicity of devices are mounted on burn-in boards, the boards are stacked in multiple stages in a temperature controlled chamber, electric current is passed through the devices, hot air with a temperature adjusted to the prescribed temperature, for example 125° C., is caused to flow parallel to the burn-in boards inside the chamber and circulate, while uniformly cooling the multiplicity of devices (see, for example, Patent References 1, 2, 3). With such a burn-in apparatus, in the case of conventional devices, the internal temperature of the devices generating heat when a current is passed therethrough is adjusted to a temperature appropriate for the burn-in test, which is about 150° C., in correspondence with the temperature of the circulating hot air, and a burn-in test of the multiplicity of devices can be conducted with good efficiency.
Furthermore, Patent Reference 3 describes, as the conventional technology, that detecting the temperature inside the chamber and maintaining it at a constant level is insufficient for directly controlling the temperature of the device itself and, therefore, a diode is formed in a hollow space of a semiconductor chip and the junction temperature of the semiconductor chip is evaluated based on the electric characteristics of the diode (see the same Patent Reference 3).
Furthermore, the Patent Reference 3 also suggests a burn-in test apparatus in which a wiring section for temperature measurements is disposed over the entire integrated circuit section of each semiconductor chip, the average temperature of the chip is detected via a connection pad provided similarly to the connection pad for current supply to the integrated circuit section, the air with adjusted temperature is supplied from air-blow fans disposed in correspondence with each semiconductor chip in a temperature adjusting apparatus, and the air flow amount supplied to each chip is controlled so that the average temperature of the chip becomes the target burn-in temperature.
On the other hand, in recent years, devices generating a large amount of heat, for example, up to about 300 W when an electric current is passed therethrough, have appeared on the market and the burn-in apparatuses have to be adapted to conduct the burn-in test thereof. However, the above-described conventional apparatuses of a general hot air circulation system cannot be adapted for the above-mentioned devices generating a large amount of heat because the air is used therein as a heat transfer medium that has to remove a large amount of heat generated by the device when an electric current is passed therethrough and the air has a low specific gravity and specific heat and can remove but a small amount of heat. For this reason, the temperature of the circulating air is reduced, the blowing speed is raised, and the amount of air is increased to increase the amount of removed heat, but in this case the size of the apparatus itself is increased, a large difference in cooling effect is observed between the upstream and downstream zones of the circulating air, and the accuracy of burn-in temperature decreases. Moreover, even if all the aforementioned measures are taken, the amount of removed heat is still limited to about 30 W. For these reasons, such devices generating a large amount of heat cannot be burn-in tested with the apparatuses of the hot air circulation system.
Furthermore, in the apparatuses where the temperature of each chip is detected and the amount of cooling air supplied to each chip is controlled, the amount of blown air can be somewhat increased and the amount of removed heat can be increased, but because the cooling medium is air, no sufficient increase in the amount of removed heat can be attained and such apparatuses similarly cannot be adapted to burn-in test the devices that generate a large amount of heat.
A temperature control apparatus of an air injection system is known as another example of the apparatuses used, e.g., for burn-in testing the devices. In such an apparatus, air injection nozzles are disposed above and below each object and low-temperature air is ejected from such nozzles (see Patent Reference 4). In this apparatus, no problems are associated with temperature distribution among the devices even when the flow speed of the air is increased to increase the flow rate thereof, but because the heat transfer medium is air, the amount of the removed heat cannot be greatly increased for the same reasons as described above, and the apparatus cannot be adapted to devices generating a large amount of heat.
A temperature test apparatus is known as a burn-in apparatus using no hot air, wherein a multiplicity of electronic components, which are devices, are carried on a printed board serving as a burn-in board, a cooling plate having circulating therein a cooling liquid that is cooled in a water-cooled heat exchanger is brought into contact with the printed board, and the electronic components are temperature tested, while being cooled if necessary (see Patent Reference 5).
With this apparatus, the amount of heat removed by cooling can be increased because heat is taken from the electronic components to boil and evaporate the liquid coolant. However, in such an apparatus, the entire bottom surface of the plate is covered with the liquid coolant and heat enters only from the portion of this surface that is in contact with the electronic device. For this reason, heat transfer at the liquid coolant side is of a perfect film boiling and evaporation mode and, therefore, a substantially increased value of heat transfer coefficient of this surface portion cannot be obtained. Another problem associated with this apparatus is that because there is a difference between the pressure inside the plate and the external pressure, which is usually the atmospheric pressure, and because the plate surface area increases since all the electronic components have to be cooled with one plate, the plate thickness cannot be decreased and heat penetration ability cannot be improved. Yet another problem is that because the plate has a flat surface with high flexural rigidity, sufficient contact with all the electronic components cannot be obtained due to the unavoidable small differences in level. Because of the aforementioned problems, cooling performance that enables a burn-in test of devices generating a large amount of heat cannot be obtained.
A cooling structure of mounted semiconductor chips is known in which each individual semiconductor chip is covered with a cap, bellows are disposed therebetween, a nozzle is disposed above each individual cap, a liquid coolant is supplied thereto from a coolant supply tube, the coolant liquid is atomized by the nozzle and blown onto the cap, the heat generated by the semiconductor chip and transferred to the cap is absorbed by the coolant particles, the particles are evaporated and released from a release vent, the gas obtained is cooled in a refrigerator to convert it into liquid, and this liquid is again supplied to the coolant supply tube, this cooling structure making it possible to remove the heat from the mounted semiconductor chips with good efficiency (see Patent Reference 6).
With this cooling structure, because the cooling liquid is atomized, the cooling liquid boils in a state close to nucleate boiling and, therefore, heat can be removed from the chip with good efficiency. However, the degree to which liquid can be atomized is limited even if liquid alone is ejected from the nozzle tip under increased pressure. For this reason, for the cooling liquid to be instantaneously evaporated to assume a nucleate boiling state when the cooling liquid is brought into contact with a cap heated by the heat generated by the chip, the difference between the temperature of the cap surface and the temperature of the cooling liquid has to be increased. For this reason, in this reference source, a perfluorocarbon with a boiling point less than about 20° C. is described as an example of the cooling liquid used.
However, if the heat transfer medium is limited to such low-boiling coolants, a limitation is placed on the types of heat transfer media that can be used. For example, heat transfer media that can be easily disposed of at low cost like water cannot be used. Other problems include that a refrigerator becomes necessary, the equipment configuration becomes complex, and the equipment cost rises.
Furthermore, a problem arising when only a coolant of one type is used to conduct latent heat cooling with a strong cooling action is that if the liquid is supplied when the operation is started, the increase in temperature caused by heat generation by the device is in a transient state and, even if the liquid is in the form of fine particles, a liquid pool appears on the cooling surface due to insufficient amount of heat generated by the device, making it impossible to conduct smooth transition to a stationary nucleate boiling state after the increase in temperature. In this case, because only a liquid coolant is pressure injected, if the pressure is reduced and the amount of injected coolant is decreased, the liquid is not atomized. The resultant problem is that the amount of liquid can be adjusted only within a narrow range and the amount of supplied liquid cannot be decreased in a temperature rise process. Another problem is the formation of liquid pools when the operation is stopped.
Furthermore, when the mounted semiconductor chip is cooled, the chip may be appropriately cooled to a low temperature such that chip operation reliability can be attained and chip endurance can be maintained. Therefore, in the stationary operation mode, a liquid coolant of one type may be used at an almost constant flow rate. However, for the cooling device to be also suitable for burn-in testing the devices that generate a large amount of heat, it has to operate so that the temperature inside the device is usually maintained at about 150° C. The resultant problem is that accurate temperature control is impossible with a coolant supply structure that has a narrow flow rate adjustment range, as described hereinabove.
[Patent Reference 1] Japanese Patent Application Laid-open No. H8-211122 (FIG. 1 and relevant explanation in the specification).
[Patent Reference 2] Japanese Patent Application Laid-open No. H11-231943 (FIG. 1 and Par. No. 25 in the specification).
[Patent Reference 3] Japanese Patent Application Laid-open No. 2000-97990 (FIG. 4 and Par. Nos. 3, 4; FIGS. 1 and 2 and relevant explanation in the specification).
[Patent Reference 4] Japanese Patent Application Laid-open No. H4-321113 (FIG. 1 and relevant explanation in the specification).
[Patent Reference 5] Japanese Utility Model Application Laid-open No. S61-114377 (FIG. 1 and relevant explanation in the specification).
[Patent Reference 6] Japanese Patent Application Laid-open No. H4-61259 (FIG. 1 and relevant explanation in the specification).